1. Field of the Invention
The present invention generally relates to circuits used in voltage-detection circuits or the like, and particularly relates to a circuit which detects an input voltage level by comparing the input voltage level with a reference voltage.
2. Description of the Related Art
When a signal input to one of the two nodes of a differential amplifier is compared with a reference voltage in ICs, a positive input node generally receives the signal, and a negative input node receives the reference voltage.
FIG. 1 is an illustrative drawing showing a signal-input configuration with regard to a differential amplifier.
A signal voltage Vs from a signal supplying circuit 201 is supplied to one input node of the differential amplifier 202. The other input node of the differential amplifier 202 receives a reference voltage Vref from a reference-voltage-generation circuit 203. The differential amplifier 202 compares the signal voltage Vs with the reference voltage Vref, and outputs a signal according to a comparison result as to which one of the signal voltage Vs and the reference voltage Vref is larger than the other. The reference voltage Vref is controlled via a reference-voltage control note. This control is aimed at adjusting the reference voltage Vref to a voltage level appropriate for detecting a signal level of the signal voltage Vs.
In the configuration of FIG. 1, the signal voltage Vs from the signal supplying circuit 201 is prone to undesirable changes because of variations in temperature and/or a power voltage level. If the changes are not commensurate with changes in the reference voltage Vref from the reference-voltage-generation circuit 203, operations of the differential amplifier 202 may suffer errors.
FIG. 2 is an illustrative drawing showing relations between the signal voltage Vs and the reference voltage Vref.
As shown on the left-hand side of the figure, the reference voltage Vref is situated at a midpoint between a HIGH level and a LOW level of the signal voltage Vs when the reference voltage Vref is appropriately adjusted. When the signal voltage Vs changes because of variations in temperature and/or a power voltage, the HIGH level of the signal voltage Vs may decline, for example, as shown in the center of FIG. 2, so as to end up having a narrow gap between the HIGH level and the reference voltage Vref. As shown on the right-hand side of FIG. 2, the LOW level of the signal voltage Vs may be lifted, for example, so as to come closer to the reference voltage Vref.
When the relationship between the signal voltage Vs and the reference voltage Vref deviates from an optimal condition as shown in the center or on the right-hand side of FIG. 2, a signal level of the signal voltage Vs may be erroneously detected. In addition, such a condition may cause a problem in that an effort to increase a signal frequency may be hampered.
A device used as the signal supplying circuit 201 is bound to have variations in circuit characteristics thereof, so that the signal voltage Vs may differ from circuit to circuit. In order to keep the reference voltage Vref at an optimal voltage, therefore, the reference voltage Vref needs to be set to a different voltage level when a different signal supplying circuit is used. Namely, a step of adjusting the reference voltage Vref needs to be incorporated into steps of building a whole circuit, thereby incurring excessive time and labor.
In order to obviate the above-identified problems, a circuit disclosed in the Japanese Patent Laid-open Application No.4-216215 uses a capacitor connection to convey only an AC component of a signal voltage, and this AC component is also added to a reference voltage to keep optimal relations between the signal voltage and the reference voltage.
FIG. 3 is a circuit diagram of this circuit which keeps optimal relations between a signal voltage and a reference voltage.
In the circuit of FIG. 3, a resistor R and a capacitor C are provided in addition to the circuit of FIG. 1. The capacitor C provides a capacitor connection for the signal voltage Vs so as to convey only an alternate-current component of the signal voltage Vs to the differential amplifier 202 by removing a direct current component. The resistor R connects between the two input nodes of the differential amplifier 202. In this manner, the signal-input node of the differential amplifier 202 is provided with a signal which is a sum of the direct-current component of the reference voltage Vref and the alternate-current component of the signal voltage Vs. As a result, the differential amplifier 202 ens up comparing the reference voltage Vref with a signal showing a variation between a HIGH level and a LOW level centering at the reference voltage Vref. An optimal voltage-level relations can thus be achieved.
The circuit of FIG. 3, however, needs a step of adjusting the reference voltage Vref for each different circuit, requiring excessive time and labor for the manufacture of the circuit. Further, use of the reference-voltage-generation circuit 203, which is comprised of variable resistors and the like, entails increases in the number of total circuit elements, a circuit size, and power consumption.
Accordingly, there is a need for a circuit configuration, used for input signals in a differential amplifier, which can keep optimal relations between a signal voltage and a reference voltage regardless of a variation in the signal voltage.
Further, there is a need for a circuit configuration, used for input signals in a differential amplifier, which does not require the reference voltage to be adjusted for each different circuit.
Further, there is a need for a circuit configuration, used for input signals in a differential amplifier, which does not use a reference-voltage-generation circuit so as to reduce the number of circuit elements, a circuit size, and power consumption.